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Power systems around the world are rapidly transitioning to much higher shares of inverter‐based resources (IBRs) with few synchronous generators remaining online. IBRs and synchronous generators have fundamentally different dynamic performance characteristics resulting in a difference in the overall power system dynamic performance. IBRs are generally more flexible and controllable than synchronous generators; however, at the same time they exhibit significantly more complex control systems. Furthermore, new and emerging capabilities are being developed progressively and in particular the so‐called grid‐forming inverters (GFM). GFM offer several new capabilities not previously possible with conventional grid‐following inverters (GFL). However, they are not well understood currently when applied in a mega scale and moving forward when they will likely take over the role synchronous generators have been performing for several decades as the workhorse of system security support. Key questions currently in the technical community include the extent to which GFM shall be similar or different to each of the synchronous machines and conventional GFL, and how various control strategies can assist in maximising the grid support capabilities sought and minimise or ideally eliminate any adverse impacts. The objective of this special issue is to provide insights into some of these unknowns. [ABSTRACT FROM AUTHOR]

A The DC distribution technology brings challenges and opportunities to the penetration of large-scale distributed renewable energy sources. In a medium-voltage DC (MVDC) distribution network, operation mode switch occurs owing to some large disturbances. To manage these disturbances and achieve seamless switch of the operation mode, a hierarchical control strategy with two layers, namely local and global layers, is proposed in this work. The novel local layer controller called P--U--I controller is designed to enhance the voltage stability, improve system controllability and suppress overcurrent under large disturbances. This controller only needs local information and does not rely on fast communication. Furthermore, as the scale of the DC distribution network expands continuously, the number of system operation mode increases in geometric progression. Therefore, a global layer controller based on breadth-first search algorithm is proposed. This controller can automatically identify the system topology and adjust the control mode of converters to optimise the system operating characteristics. Thus, this hierarchical control strategy can accurately control the system power in steady state, suppress overcurrent under large disturbances and suit large-scale DC distribution networks. Finally, a three-terminal MVDC distribution network simulation model established on power systems computer-aided design and RT-LAB validates the proposed strategy. [ABSTRACT FROM AUTHOR]

Focusing on high-efficiency sequential feature learning for short-term voltage stability (SVS) assessment, this study develops a light-duty time series (TS) learning machine based on symbolic TS datasets, namely wordbooks. Numerous cumbersome TS acquired from post-contingency synchrophasor measurements are first tactfully transformed into short symbolic words, constituting a compact and light wordbook. By centralising the intra-class words in the wordbook, a series of keywords are quickly extracted without iteration to perform wordbook learning. Owing to the portability of the wordbook, online SVS assessment models can be derived extremely fast from it. With such merits, wordbook learning is designed to be periodically conducted by studying new cases from up-to-minute measurements, resulting in enhanced adaptability to inconstant unknown situations. Fractional affixes that frequently occur in the wordbook are further extracted for pattern discovery of voltage instability. Test results on the realistic Hong Kong power grid illustrate the effectiveness and advantages of the proposed learning machine. [ABSTRACT FROM AUTHOR]

Short-term voltage stability (STVS) issue poses a significant threat to receiving-end power systems with high penetration of induction motors. To improve STVS of a receiving-end system in a limited geographic area, this study proposes a local-area STVS control system (LVCS). LVCS consists of phasor measurement units (PMUs), a local-area centralised controller (LCC), generator-level actuators (GLAs) and high-speed communication networks. The PMUs or GLAs are pre-deployed according to the observability or controllability of STVS. LCC receives and processes voltage measurements from PMUs to generate supplementary excitation control signals (SECSs). The SECSs are next forwarded to and executed by GLAs. Simulation studies on a practical receiving-end system in China Southern Power Grid demonstrate that: once the target system suffers a serious fault, LVCS can 'feel' the voltage sag at critical buses and enforce the excitation control of generators to inject more dynamic var into the receiving-end system. As a result, the increasing var demand is met locally and STVS can be improved considerably. Time delay caused by synchrophasor transmission has also been considered in the design of LVCS to make it feasible in practice. Thus, the proposed LVCS offers a new and effective solution to the STVS problem. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses various reports within the issue on topics including dynamic synchrophasor estimation, voltage controllability and renewable energy.